Coal and oil-fired boilers and the like can generate large quantities of sulfur oxides because of the significant quantities of sulfur usually contained within the fuels utilized by these devices. These sulfur oxides become entrained in the flue gases that are generated in the combustion process and are emitted to the atmosphere. Once air-borne, these contaminants react with the moisture in the atmosphere and produce what is commonly referred to as acid rain. Acid rain, which is sulfuric acid, is released from the atmosphere and falls back to the earth where it becomes entrained in the groundwater, streams, lakes, and other water reservoirs. Environmental legislation restricts the emission of these pollutants into the atmosphere because of the long-term health effects that have been linked to significant exposure to these contaminants.
Current state of the art technology, limestone based wet flue gas desulfurization, incorporates the use of a calcium-based reagent to neutralize the sulfur oxides. The calcium in the limestone reacts with the sulfur oxides and produces a calcium sulfate product, commonly known as gypsum. Typically, the gypsum byproduct is disposed of in a landfill, used as filler in concrete or used in the production of wallboard. Disposal in a landfill requires a large initial capital investment as well as significant resources to monitor and maintain the landfill throughout the life of the plant. The use of the gypsum as concrete filler or wallboard product requires a significant transportation cost to deliver the gypsum to the manufacturing facility. The negative aspects render these two alternatives economically unattractive as potential disposal solutions. Existing limestone based wet flue gas desulfurization technologies are expensive to operate and they create a solid waste disposal problem or a low value by-product. They also utilize slurry for scrubbing and are subject to the associated high maintenance costs and operational difficulties.
Reagents other than limestone have also been used in absorption processes to form byproducts with more economical utilization. Ammonia may be used as the reagent and the byproduct is ammonium sulfate. The reaction of ammonia, sulfur oxide and oxygen produces ammonium sulfate. The major use of ammonium sulfate is as a fertilizer, which accounts for approximately 95% of the domestic ammonium sulfate consumption. The advantage of using ammonium sulfate over other fertilizers is the presence of both nitrogen and sulfur, which is ideal for highly alkaline soils. Sulfur deficient soils are becoming more common because of larger crop yields removing increased amounts of sulfur, continued use of fertilizers which contain no sulfur, and reduced sulfur disposition from the atmosphere due to reduced sulfur emissions from coal and oil fired power plants. Sulfur is a valuable nutrient because it has been closely linked to nitrogen efficiency, it aids in the plant seed production process, and it supports the chlorophyll production process.
Although ammonia is a well-known reagent used in the wet fuel gas desulfurization process, the ammonia based aerosols generated in the sulfur oxide absorption process restrict the efficient operation of the many processes that are available. To achieve an efficiently operating ammonia based wet flue gas desulfurization system design, the pH of the ammonia based wet flue gas desulfurization system is critical. Higher pH values lead to increased sulfur oxide removal efficiency and thus, higher production rates of ammonium sulfate. However, operation at higher pH values results in higher ammonia vapor pressure. The ensuing gas phase ammonia reacts with gas phase hydrogen chloride and sulfur dioxide forming aerosols. The highly visible ammonia slip and aerosols are considered secondary pollution and produce a blue haze or plume at the discharge of the flue gas stack. The ammonia based aerosols are submicron in size and are difficult to remove in the sulfur dioxide absorption process. Gaseous ammonia at even low concentrations in the flue gas results in these visible plumes and increased plume opacity. Current ammonia based wet flue gas desulfurization technologies are vulnerable to these visible emissions which have adverse health effects.
Other problems encountered by existing ammonia based wet flue gas desulfurization technologies which employ prescrubber are due to crystallization of the ammonium sulfate product occurring in the prescrubber. Product contamination by fly ash and other impurities results from difficulties with product separation. Product purity decreases as a consequence. Another result of crystallization in a prescrubber is that scrubbing is accomplished with a slurry. Slurry based scrubbing leads to high maintenance costs and operational difficulties. Prescrubbers also result in a larger pressure drop for the waste gas through the ammonia based wet flue gas desulfurization system.
An object of the present invention is to provide an improved process and system for the absorption of sulfur oxides from flue gases and other sulfur oxide-containing gases. A specific object is the reduction of the emission of ammonia and sulfur compound aerosols. The invention particularly involves the scrubbing of the sulfur oxides from the gases with an ammonium sulfate liquor in a countercurrent spray tower absorber and passing the scrubbed gases to a wet electrostatic precipitator where ammonia aerosols, sulfur trioxide aerosols and ammonia slip are precipitated out of the scrubbed gases. In the preferred embodiment, the ammonium sulfate liquor with the absorbed sulfur oxides is passed from the spray tower into a separate reaction tank where ammonia and air are injected. The ammonia and sulfur oxides react to form ammonium sulfite which is oxidized to the sulfate by the oxygen in the injected air. The gases and vapors from the separate reaction tank are separately scrubbed before return of the gases to the absorber. The absorber construction with by-pass eliminating rings improves the sulfur oxide absorption and reduces aerosol formation and emission.